title: Microbial alignment in flow changes ocean light climate.
abstract: The growth of microbial cultures in the laboratory often is assessed informally with a quick flick of the wrist: dense suspensions of microorganisms produce translucent "swirls" when agitated. Here, we rationalize the mechanism behind this phenomenon and show that the same process may affect the propagation of light through the upper ocean. Analogous to the shaken test tubes, the ocean can be characterized by intense fluid motion and abundant microorganisms. We demonstrate that the swirl patterns arise when elongated microorganisms align preferentially in the direction of fluid flow and alter light scattering. Using a combination of experiments and mathematical modeling, we find that this phenomenon can be recurrent under typical marine conditions. Moderate shear rates (0.1 s(-1)) can increase optical backscattering of natural microbial assemblages by more than 20%, and even small shear rates (0.001 s(-1)) can increase backscattering from blooms of large phytoplankton by more than 30%. These results imply that fluid flow, currently neglected in models of marine optics, may exert an important control on light propagation, influencing rates of global carbon fixation and how we estimate these rates via remote sensing.
authors.affiliation: Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
authors.lastname: Marcos
authors.forename: Justin R
authors.initials: JR
authors.lastname: Seymour
authors.forename: Mitul
authors.initials: M
authors.lastname: Luhar
authors.forename: William M
authors.initials: WM
authors.lastname: Durham
authors.forename: James G
authors.initials: JG
authors.lastname: Mitchell
authors.forename: Andreas
authors.initials: A
authors.lastname: Macke
authors.forename: Roman
authors.initials: R
authors.lastname: Stocker
mesh_terms: D002980:Climate; D008027:Light; D015233:Models, Statistical; D009792:Oceans and Seas; D012542:Scattering, Radiation; D014871:Water Microbiology